Go here: http://www.tombraiderforums.com/showthread.php?t=151774
and scroll down.
There is everything about flipmaps and OCB lists and numbers. And a lot more as you can see
Also take it easy and don't run before you can walk, it is a lot of info and we all had to go through it.

How do I use the Back to Basics WADs, such as the Persia or Northern Legends WAD with TREP? I read that BtB uses TREP tomb4.exe for the competition, but it doesn't seem to be included in the package. I manually patched tomb4.exe with TREP with all the engine improvement patches I could find (vertices limit, extended exe, etc.) to accommodate for the HD objects in the WADs, but when I try to run a level build with the BtB WAD and compiled with NG_tom2pc it crashes during loading in TREP tomb4.exe. It only works in the TRNG tomb4.exe, but I want to use TREP instead...

Do I need to use a specific version of NGLE or NG_tom2pc?
The standard tom2pc crashes when compiling files built with the BtB WADs...

EDIT:
Seems I have managed to solve the problem - turns out I wasn't applying all required patches in TREP after all. I didn't apply the "remap scene memory" patch, because the annotation said it was for fixing high-DD crashes, so I somehow assumed it didn't have much to do with objects, but once I activated it I could also check "increase statics limit" patch, which I think resolved the issue and now the level built with the Northern Legends WAD loads fine.

*.bat file is the best solution I think. Just make a text file which contains this:
tomb4.exe setup
Then save file and change extension. This is the shortcut for any computer as long as it is in game folder.

Any general advice on making hollowed-out transparent structures? I'm looking to make some 4-tile transparent pyramid shapes like in this screenshot from The Rescue 6 below:

I've tried on my own and am having some trouble. The inside portion of these structures is a separate room, correct? My trouble is coming from trying to shape that "inner room," my attempts seem to all result in just making the "exterior" of the pyramid. If anyone has suggestions or know some source on how to make these it would be appreciated!

No. Incorrect, as a matter of fact.
The pyramids are not separate rooms, in fact only two rooms are shown in the screenshot: the one containing the pyramids, and the one you can see through them below. The pyramids are made from raised triangular floor sectors in the upper room; there is, technically speaking, no inside, because there are no faces on the inside, so the exterior is all you get to work with.
If you want to create the illusion of interior faces (or ‘texture the interior’), you’ll need to apply your textures on the exterior faces with the ‘double-sided’ (and ‘transparent’) modes activated, as you would with a water surface.
Any texture on the floor of such a pyramid needs to be on a portal to the room below, so in order to have a face there for you to texture, that portal must have Toggle Opacity activated as well. (Obviously, you’ll have to texture this one from the room below).

Any texture on the floor of such a pyramid needs to be on a portal to the room below, so in order to have a face there for you to texture, that portal must have Toggle Opacity activated as well. (Obviously, you’ll have to texture this one from the room below).

This was the part that had me struggling! I didn't realize on which plane the floor of this pyramid was supposed to go, so my early efforts ended up with a black "untextured" floor. This helped clear it up and I have it working. Thank you, Mulf.

Guys which TREP patch is the inventory background color?
Not speaking about custom images, just the solid color.
EDIT Nvm found it, "Clear background images".

Instead, how do I use TREP enemy patcher to make the mutant mortal?
I tried the blood flag with 150 HP but I get the "idle mutant" error.
I mean that state when he stops attacking / being targetted but doesn't die.

EDIT Found it. I had to apply the extra "exploding enemies" patch.
And set all 5 slots to 63 since no other enemy in this game explodes.

I also figured out something much more important:
TREP patch "clear menu backgrounds" has more effect than skipping inventory lag
it actually prevents win7 from disabling aero ("color scheme has changed to basic")
as follows it keeps aero v-sync and prevents screen tearing which would otherwise occur.
If you build with TREP you should always, always use it.